Thin film transistors are used in a variety of different applications, such as in metal oxide semiconductor Static Random Access Memory (SRAM) fabrication processes, and flat panel liquid crystal displays, as well as many other applications. These are frequently formed from polysilicon. The grain boundaries of polysilicon thin film transistor bodies normally have dangling bonds. These dangling bonds can trap and release electrons during thin film transistor operation. This affects the thin film transistor threshold voltage and reduces Ion/Ioff ratio.
In certain applications a lower Ion/Ioff ratio can be tolerated. In other operations it cannot, and the thin film transistor is hydrogenated to eliminate these dangling bonds and establish a higher Ion/Ioff ratio. Unfortunately, the deposited hydrogen tends to migrate with time, and the dangling bonds in effect reappear. Eventually, this ruins the transistor, shortening the life time of the product.
Such hydrogenated thin film transistors are well known. Generally, these are formed by depositing a layer of silicon dioxide over the gate electrodes. A polysilicon layer is then deposited and etched to form the transistor body which will become the channel, source, and drain of the transistor after source/drain implant. This is then coated with a silicon dioxide layer. Subsequent to the deposition of the second silicon dioxide layer and the source/drain implant, the polysilicon is hydrogenated by exposing the wafer to a hydrogen plasma, or by implanting hydrogen ions into the polysilicon layer. This is then coated with a thicker layer of silicon dioxide or a layer of silicon nitride. Contacts for metal to connect the gate electrode layer are then opened using a photolithography technique, followed by deposition, photo printing and etching of a metal layer as transistor terminal leads.
Such a prior art transistor is shown in FIG. 1 which is a cross-sectional view of a prior art thin film transistor 50. The substrate 51 includes interconnect lines 52 and 53 and gate electrode 54. As shown, the transistor further includes a heavily doped drain and a lightly doped drain 56 adjacent the channel of the transistor 57. This is directly above the gate dielectric 58. Lead 59 for the source of the transistor connects through a contact opening to interconnect line 52 which in turn connects through a buried contact to the extension of the source electrode of the transistor. The metal lead 60 for the drain connects to a drain electrode through interconnect line 55 in the same manner, while the metal lead for the gate (out of the plane and not shown) connects to the extension of the gate electrode through a contact opening. It should be noted that connection of the source, the drain, and the gate electrode to other elements of the circuit do not have to go through the metal layer and can be accomplished through the use of interconnect lines on the gate electrode layer or the extension of these electrodes themselves.
With this process the hydrogen ions that have been bonded to the dangling bonds of the polysilicon transistor body layer can migrate out of the polysilicon transistor body layer as previously indicated. Over time this can cause the transistor to fail.